In the semiconductor manufacturing industry, high-temperature furnaces are used to create semiconductor junctions and circuits by exposing semiconductor materials to chemicals under high temperature. To regulate the desired chemical reactions, and to ensure consistent quality, accurate control of furnace temperature is required. The furnace temperature may exceed 1000 degrees Centigrade.
Control may be achieved by creating a feedback loop in which a furnace temperature regulator is coupled to a temperature sensor in the furnace. The sensor detects internal furnace temperature and produces an output electrical signal proportional to the temperature. The signal is coupled to a computer, or to a temperature display, enabling either manual or automatic monitoring and adjustment of the furnace.
Conventionally, such temperature sensors comprise a precious metal thermocouple having a junction of two different precious metals or alloys. Heat applied to the function of the two metals causes a thermo-electric potential difference to develop which produces an output signal. Precious metals used in conventional thermocouples are intrinsically temperature resistant, i.e. their characteristics are not affected by exposure to high temperature, but have numerous disadvantages including extremely low output voltage, high cost, susceptibility to electromagnetic interference, low signal-to-noise ratio, and slow response time.
To overcome these disadvantages, optical fiber thermometers (OFTs) have been developed. An optical fiber thermometer system may comprise a black body radiator on an optical fiber having a "hot" end in a furnace under test and a "cold" end coupled to receiving and decoding electronics. The "cold" end of the fiber is coupled, outside the furnace under measurement, to a photodiode receiver assembly which includes amplification and temperature conversion electronics. Several fibers can be bundled to increase signal strength or to enable temperature sensing at several distributed locations in the furnace.
Most conventional optical fibers, such as those used in telephony and computer data transmission, become brittle or will melt in the high temperatures of semiconductor furnaces. Therefore, some known devices employ sapphire fibers or rods, which are highly heat-resistant and which provide good optical qualities. For example, U.S. Pat. Nos. 4,576,486, 4,750,139, and 4,845,647 disclose OFT systems using a high temperature sapphire fiber in a furnace coupled to a low-temperature silica fiber outside the furnace. Also, U.S. Pat. No. 3,626,758 shows a sapphire fiber for high-temperature sensing having a metallic coating sputtered on the tip which serves as a blackbody radiator.
However, optical quality Sapphire fibers are not commercially available, and sapphire rods are expensive, costing $1000 per foot or more. Sapphire rods are not flexible and thus have limited usefulness. Nevertheless, the temperature resistance of sapphire fibers makes them a desirable alternative for some applications.
Silica fiber is usually commercially available at $1 per foot or less, and is highly flexible. Unfortunately, harsh temperature and chemical conditions in a semiconductor furnace quickly cause silica fibers to become brittle and useless. Semiconductor furnaces can include large quantities of hydroxyl ions in the furnace atmosphere, and these ions can penetrate into the fiber and cause severe embrittlement in a very short time, even at temperatures well below the melting point of silica.
An article by J.P. Dakin and D.A. Kahn, "A novel fibre-optic temperature probe," Optical and Quantum Electronics 9 (1977), p. 540, proposes use of an OFT with a single silica fiber having a maximum operating temperature of 1,100.degree. C. for direct temperature measurement. The fiber is encased in a fine stainless steel tube. However, the tube does not provide a hermetic seal and the reference does not disclose how to coat or treat the fiber to protect against embrittlement by hydroxyl ions. Moreover, the stainless steel tube suggested is inflexible.
U.S. Pat. No. 4,794,619 (Tregay) shows a thermally emissive surface implanted in a low-temperature optical fiber which may comprise glass. However, Tregay does not disclose how to protect the fiber from extreme temperatures or embrittlement. Moreover, external light can enter the fiber around the outer diameter of the implanted emissive material.